The alternative pathway of complement activation provides nonimmune recognition of microorganisms, virus-infected and malignant mammalian cells leading to host defense reactions of opsonization. cytolysis, and inflammation. Particulate activators share the ability to provide sites for covalent C3b binding which are protected from serum regulatory proteins. In addition several intrinsic membrane proteins have been identified which regulate complement activation and C3 breakdown and Prevent activation by humanm cells in human serum. Model lipid membranes in the form of liposomes can activate complement by the alternative pathway. Since the composition of liposomes is defined and easily modified, they provide a simple model with which to study the role of membrane lipids and proteins in the formation and regulation of the C3 convertase. These studies will focus on the role of the polar head group of the phospholipid in providing C3 binding sites and on the effects of changes in membrane fluidity on C3 binding and breakdown. Regulation of activation by the membrane regulatory proteins, decay-accelerating factor (DAF) and membrane cofactor protein (MCP) will be tested by insertion of these into activating liposomes. In addition, the role of Epstein-Barr virus (EBV) glycoproteins in alternative pathway activation and regulation will be studied. The molecules involved and characteristics of an EBV- associated C3 regulatory activity will be examined by identifying C3 binding molecules in EBV and EBV-infected cell membranes. The role of antibody in alternative pathway activation will be studied using EBV-infected cells. One of the viral envelope glycoproteins, gp350, has been identified as an alternative pathway activator and antibody specific for gp350 enhances alternative pathway activation by EBV-infected cells. Purified gp350 inserted into liposomes and monoclonal antibodies to gp350 will be used to determine the ability of antibody to enhance activation by cross-linking or providing additional C3 binding sites. The influence of DAF on alternative pathway activation by EBV-infected cells and gp350 liposomes will be examined as well. These studies will increase our knowledge of the membrane determinants of the alternative complement pathway recognition system and add to our understanding of host defense against bacterial and viral infection.